Remove unneeded imports
Make HandlersByApplicationID store Executors

HandlersByApplicationID now stores MessageHandlerWithExecutors, so that
an Executor can be assigned to a MessageHandler. There are two
implementations of MessageHandlerWithExecutor:
SimpleMessageHandlerWithExecutor which simply stores a MessageHandler and
an Executor and MessageHandlerWithDefaultExecutor which stores a
MessageHandler and returns an Executor that runs code in the calling
Thread.
Make ClientMessageDispatcher use Executors

ClientMessageDispatcher will now do as much work as possible in a
Runnable executed by the Executor that was set for the MessageHandler.
Implement addHandler with Executor in TCPConMan
Update to Netty 4.0.13

new AttributeKey has been deprecated in favor of AttributeKey.valueOf,
code has been changed accordingly